Metal Oxide Transistors via Polyethylenimine Doping of the Channel Layer: Interplay of Doping, Microstructure, and Charge Transport

Huang, Wei; Zeng, Li; Yu, Xinge; Guo, Peijun; Wang, Binghao; Ma, Qing; Chang, Robert P. H.; Yu, Junsheng; Bedzyk, Michael J.; Marks, Tobin J.; Facchetti, Antonio

doi:10.1002/adfm.201602069

Cited by 79 publications

(125 citation statements)

References 69 publications

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“…[26] In another report, Huang et al showed that PEI can n-dope and tune the performance of In 2 O 3 -based www.advelectronicmat.de Adv. [27] These results are consistent with our finding that PEI is a strong n-type dopant. Mater.…”

Section: Wileyonlinelibrarycomsupporting

confidence: 93%

n‐Type Doping Induced by Electron Transport Layer in Organic Photovoltaic Devices

Wang

Zhang

et al. 2017

Adv Elect Materials

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into the active layer, [6,7] the background doping, typically p-type, is believed to arise from polymer synthesis and is difficult to control. [3,9] In a recent report, we demonstrate that OPV active layers deposited on top of a MoO x hole transport layer (HTL) are p-doped, with the background hole concentration depending on the work function of MoO x and the active layer material. [8] Here we show that the electron transport layer (ETL) could also induce doping in the OPV active layer. In particular, poly(3hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) films deposited on top of branched polyethylenimine (PEI) ETL convert from typical lightly p-doped to strongly n-doped, with devices displaying substantially lower short-circuit current density (J sc ) and a strikingly different external quantum efficiency (EQE) spectral shape when compared with devices made on ZnO ETL. Using drift-diffusion and transfer matrix method (TMM) simulations to elucidate the experimental results, we establish that a combination of carrier type and concentration in the active layer fully explain the observed J sc and EQE spectral differences. Finally, we show that the origin of n-doping is due to the high solubility of PEI in the active layer processing solvent. The understanding is confirmed by device behaviors for PEI and another polymeric ETL, polyethylenimine ethoxylated (PEIE), and the dependence on active layer thickness and the energy of negative integer charge-transfer state. This research provides a comprehensive understanding of the effects of doping type and concentration on OPV device behaviors. Specifically, it provides an explanation for the wide variation of EQE spectra reported in the literature for OPV devices made with the same active layers but different interfacial contact layers. [10,11] Results and DiscussionsPEI and ZnO have been reported to be good ETL candidates in OPVs. [12,13] ZnO is an effective metal oxide ETL in OPV because of its low work function that enhances built-in field and deep valence band maximum that blocks holes to the cathode. [13,14] On the other hand, PEI is a polymeric ETL that functions as an interfacial modifier to reduce the cathode work function. [12] Figure 1a and Table 1 show the device current density versus Doping in an organic photovoltaic (OPV) device can largely impact its performance. In this work, it is discovered that n-type electrical doping in the poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester active layer can be induced by a certain electron transport layer (ETL), particularly branched polyethylenimine (PEI). Consequently, OPV devices with different ETLs exhibit dramatically different current density-voltage behaviors and external quantum efficiency (EQE) spectra. Using drift-diffusion modeling, Hall effect, and capacitance-voltage measurements, it is shown that the difference in EQE spectra originates from the different background carrier type and concentration in the OPV active layer, dictated by the specific properties of ETL. Fa...

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Section: Wileyonlinelibrarycomsupporting

confidence: 93%

n‐Type Doping Induced by Electron Transport Layer in Organic Photovoltaic Devices

Wang

Zhang

et al. 2017

Adv Elect Materials

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“…Nevertheless, solution‐processing of MO precursors holds significant promise for lower cost production and compatibility with flexible substrates . In this regard, indium oxide (In 2 O 3 ) is one of the most investigated solution‐processed oxide semiconductors, however, the carrier density of pristine In 2 O 3 is difficult to control and In 2 O 3 films are usually polycrystalline, limiting their performance uniformity over large areas as well as mechanical flexibility . To address both of these issues, metals such as Zn and/or Ga are added to In 2 O 3 , enabling the growth of amorphous MO thin films with enhanced electronic properties and ultimately affording far more stable TFT characteristics …”

Section: Performance Metrics Of the Indicated Tft Devices On Si/sio2 mentioning

confidence: 99%

Polymer Doping Enables a Two‐Dimensional Electron Gas for High‐Performance Homojunction Oxide Thin‐Film Transistors

et al. 2018

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deposition (PLD), and radiofrequency sputtering. [2,[11][12][13] Nevertheless, solutionprocessing of MO precursors holds significant promise for lower cost production and compatibility with flexible substrates. [2,9,14,15] In this regard, indium oxide (In 2 O 3 ) is one of the most investigated solution-processed oxide semiconductors, [7,[16][17][18][19][20][21] however, the carrier density of pristine In 2 O 3 is difficult to control and In 2 O 3 films are usually polycrystalline, limiting their performance uniformity over large areas as well as mechanical flexibility. [22] To address both of these issues, metals such as Zn and/or Ga are added to In 2 O 3 , enabling the growth of amorphous MO thin films with enhanced electronic properties and ultimately affording far more stable TFT characteristics. [22][23][24][25][26] Recently, we reported a new strategy for high performance, near-zero threshold voltage (V Th ), bias stress-stable, and amorphous In 2 O 3 films and TFTs by simply doping the In 2 O 3 with an electrically insulating polymer such as poly(4-vinylphenol) (PVP) or polyethylenimine (PEI) to afford amorphous MO:polymer blend semiconducting nanocomposites. [2,17,22] The attraction of electron-rich PEI versus PVP is that the TFT High-performance solution-processed metal oxide (MO) thin-film transistors (TFTs) are realized by fabricating a homojunction of indium oxide (In 2 O 3 ) and polyethylenimine (PEI)-doped In 2 O 3 (In 2 O 3 :x% PEI, x = 0.5-4.0 wt%) as the channel layer. A two-dimensional electron gas (2DEG) is thereby achieved by creating a band offset between the In 2 O 3 and PEI-In 2 O 3 via work function tuning of the In 2 O 3 :x% PEI, from 4.00 to 3.62 eV as the PEI content is increased from 0.0 (pristine In 2 O 3 ) to 4.0 wt%, respectively. The resulting devices achieve electron mobilities greater than 10 cm 2 V −1 s −1 on a 300 nm SiO 2 gate dielectric. Importantly, these metrics exceed those of the devices composed of the pristine In 2 O 3 materials, which achieve a maximum mobility of ≈4 cm 2 V −1 s −1 . Furthermore, a mobility as high as 30 cm 2 V −1 s −1 is achieved on a high-k ZrO 2 dielectric in the homojunction devices. This is the first demonstration of 2DEG-based homojunction oxide TFTs via band offset achieved by simple polymer doping of the same MO material. Thin-Film TransistorsThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.As a consequence of their outstanding charge transport properties and excellent optical transparency, metal oxide thin-film transistors (MOTFTs) have been heavily investigated for applications in state-of-the-art flat panel display technologies and next-generation electronics. [1][2][3][4][5][6][7][8][9][10] To date, several growth techniques have been utilized to fabricate MO films for TFTs including atomic layer deposition (ALD), pulsed-laser

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“…PEI has been applied as an effective n‐type dopant to increase the mobility of n‐type metal oxide and polymer in FETs. [54a,65] PEI is introduced to dope metal oxide and indium oxide (In 2 O 3 ) to enhance the transistor performance especially the mobility. PEI doping of In 2 O 3 films not only impedes crystallization and controls the carrier concentration, but also acts as electron dopants and/or scattering centers.…”

Section: Applications In Optoelectronic Devicesmentioning

confidence: 99%

“…Except for the metal oxide, PEI plays a more apparent role in doping the polymer semiconductor. [65c] Nanopores enhanced n‐doping of polymer poly(diketopyrrolopyrrole‐thiophene‐thieno[3,2,b]thiophenethiophene) (DPP2T‐TT) doped by PEI has been demonstrated. Doping with PEI at a high concentration enhances the n‐channel characteristics via passivating the p‐channel behavior.…”

Section: Applications In Optoelectronic Devicesmentioning

confidence: 99%

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Low Work Function Surface Modifiers for Solution‐Processed Electronics: A Review

Peng

Qin

et al. 2018

Adv Materials Inter

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electron injection or extraction, whereas a high work function metal such as Au is employed for hole injection or extraction. Since the low work function metallic electrodes are normally chemically reactive, they are readily oxidized in air (oxygen) and water when serving as cathodes. Airstable electrodes with a low work function are more challenging compared to ones with a high work function. Figure 1 shows the physics of band bending when a metal electrode is in contact with an n-type semiconductor to illustrate the importance of a low work function for the electrode. When the work function (WF = E vac − E F ) of the metal electrode is larger than that of the n-type semiconductor, electron flows from the semiconductor to the metal electrode. Unfavorable band bending for electron collection is formed at the interface in this case. By contrast, if the work function of the metal electrode is lower than that of the n-type semiconductor (Figure 1b), the opposite band bending is formed to favor electron collection from the semiconductor to the electrode. Therefore, an electrode with a sufficiently low work function is crucial to efficient electron injection or collection. Surface modification has been shown to be effective in reducing the work function of the electrodes to facilitate electron injection and collection, [7] and enhanced electron collection or injection improves the performance of organic electron devices. The proper surface modification strategy can produce air-stable electron collection or injection electrodes combined with high work function electrode/surface modifiers. This approach also enables diverse design of novel-device architectures for flexible electronics and printed electronics.Several types of low work function surface modifiers have been developed for the cathode interface. Inorganic metal oxides and alkali metal salts are effective low work function modifiers. N-type metal oxides such as titanium oxide (TiO x ) [8] and zinc oxide (ZnO) [9] are often used as cathode interlayers due to the excellent charge mobility and electron selectivity. The metal oxide film with a typical thickness of more than 10 nm possesses a low work function when coating on a high work function electrode and high-temperature annealing is normally required. Alkali metal salts like LiF, [10] Cs 2 CO 3 , [11] and Li 2 CO 3 [12] can also be incorporated into the cathode interlayer to reduce the electrode work function. Considering the intrinsic insulating properties, the thickness must be carefully controlled to ensure efficient electron extraction and injection. Another example is organic molecules grafted with ionic or polar Interfaces with a low work function are crucial to electron injection and collection in semiconducting devices such as diodes and transistors, and many types of surface modifiers have been developed to tailor the work functions of electronic devices. In this review, the basic and working mechanisms of low work function surface modifiers covering the surface dipole, low intrinsic bulk work funct...

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Metal Oxide Transistors via Polyethylenimine Doping of the Channel Layer: Interplay of Doping, Microstructure, and Charge Transport

Cited by 79 publications

References 69 publications

n‐Type Doping Induced by Electron Transport Layer in Organic Photovoltaic Devices

n‐Type Doping Induced by Electron Transport Layer in Organic Photovoltaic Devices

Polymer Doping Enables a Two‐Dimensional Electron Gas for High‐Performance Homojunction Oxide Thin‐Film Transistors

Low Work Function Surface Modifiers for Solution‐Processed Electronics: A Review

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